How well children perform in the classroom could be linked to how physically active they are, suggests a Dutch review of previous studies.Writing in Archives of Pediatrics & Adolescent Medicine, researchers said they found strong evidence of a link between exercise and academic performance.The review looked at 14 studies involving more than 12,000 children.Exercise may help by increasing blood and oxygen flow to the brain, it said.But the authors of the study, from VU University Medical Center in Amsterdam, said more accurate and reliable measurement instruments were needed to examine the link in greater detail.Dr Amika Singh and colleagues were prompted to look at the relationship between physical activity and academic performance because of concerns that pressure to improve children's school marks could mean they spend more time in the classroom and less time doing physical activity.So the authors identified 10 observational and four interventional studies for review.Twelve of the studies were conducted in the United States, one in Canada and one in South Africa.Sample sizes ranged from 53 to about 12,000 participants between the ages of six and 18 years.The period of follow-up varied between eight weeks and more than five years.Two of the studies reviewed were rated as being of high quality, the study says.Blood flowResearchers said they found strong evidence of a "significant positive relationship" between physical activity and academic performance using those two studies as evidence.The study said this could be because exercise helps cognition by increasing blood and oxygen flow to the brain.Physical activity could also reduce stress and improve mood, making children more likely to behave in the classroom.Dr Singh said: "Children who learn to participate in sport also learn to obey rules. This may mean they are more disciplined and able to concentrate better during lessons."The researchers said more studies examining the exact relationship between physical activity and academic performance were needed."People always ask, 'How much exercise do I need to do to get an A?' We don't know that but we would like to find out," said Dr Singh."Children should be active for at least one hour a day, for health reasons. But we also need to look at other things, like what kind of activities they should do, when they should do them and for how long."Reliable and valid measurement instruments were also required to assess the relationship accurately, the study added.No study in their systematic review used an objective measure of physical activity. Many of the studies required children or their parents to note down how much exercise they were doing.